This invention relates to the process for producing 1,4-butanediol and tetrahydrofuran by hydrogenation of dimethylsuccinate in the presence of a copper chromite catalyst wherein the temperature of the reaction is controlled to obtain a reaction product mixture with a predetermined mole ratio of 1,4-butanediol and tetrahydrofuran. This invention also relates to a method of separating 1,4-butanediol and tetrahydrofuran from a mixture containing other components such as methanol and water.
Tetrahydrofuran is a well-known solvent and an intermediate in the manufacture of 1,4-dichlorobutane, nylon, and polyurethanes. Dehydration of 1,4-butanediol produces tetrahydrofuran. Other reaction products from 1,4-butanediol include acetylenic chemicals such as gamma-butyrolactone, pyrrolidone, alkyl pyrrolidones, vinyl pyrrolidone, and poly-vinyl pyrrolidone. Butanediols are also useful in the production of polymeric materials such as polybutylene terephthalate. Almost all of the butanediol on the market today is obtained via butynediol which is produced from acetylene and formaldehyde by the Reppe synthesis. Another method of producing 1,4-butanediol is the hydrolysis of 1,4-dichlorobutene to 1,4-butenediol which is then hydrogenated to butanediol.
U.S. Pat. No. 2,772,291 and 2,772,292 describe the hydrogenation of maleic anhydride in the presence of nickel or cobalt catalysts to produce mixtures containing tetrahydrofuran. Neither patent discloses the use of copper chromite in the hydrogenation of a succinate ester. U.S. Pat. No. 2,772,293 also discloses a method of hydrogenating maleic anhydride to tetrahydrofuran using nickel or cobalt based catalysts. The patentees note that when the process was attempted with a copper chromite catalyst, conversion to tetrahydrofuran was not detected.
U.S. Pat. No. 4,301,077 discloses a process of hydrogenating oxygenated C.sub.4 hydrocarbons in the presence of water and a ruthenium-containing catalyst. Although tetrahydrofuran and 1,4-butanediol are obtained from the process, the use of a copper chromite catalyst is not disclosed.
U.S. Pat. No. 4,048,196 discloses the preparation of an intermediate, gamma-butyrolactone which is subsequently converted into butanediol and/or tetrahydrofuran using a nickel catalyst. British Patent No. 1,512,751 discloses the preparation of 1,4-butanediol from gamma-butyrolactone using a copper oxide, chromium oxide catalyst which has been pretreated with hydrogen. The process of this invention is a method of producing mixtures of tetrahydrofuran and 1,4-butanediol in a one-step reaction without the separate preparation of gamma-butyrolactone as an intermediate.
A multistage process for producing 1,4-butanediol from maleic anhydride is disclosed in U.S. Pat. No. 4,361,710. In contrast to the process of this invention, the process of the patent requires the washing of the mixture containing maleic anhydride with an aliphatic alcohol of not less than 8 carbon atoms with subsequent heating to form a solution of maleic diester in the alcohol. The production of tetrahydrofuran is also disclosed when the residence time of the reactants in the reactor is increased with the amount of tetrahydrofuran increasing up to 25% of the butanediol. The process of the present invention can produce mixtures in which the mole ratio of tetrahydrofuran to butanediol is greater than 2.
The hydrogenation of mono- and dialkyl esters is taught in U.S. Pat. No. 2,091,800. Dialkyl esters are hydrogenated using a copper chromite catalyst at 250.degree. C. but there is no mention of tetrahydrofuran as a product of the reaction. In fact, the hydrogenation product of diethyl succinate at 250.degree. C. is reported as tetramethylene glycol in a yield of 80.5%. Canadian Patent No. 1,037,487 discloses that tetrahydrofuran has been prepared from esters such as succinic acid esters by hydrogenation over a hydrogenation catalyst. However, the reference does not disclose any specific hydrogenation catalyst. One skilled in the art would not expect the copper chromite catalyst required in the process of this invention to produce tetrahydrofuran in any significant amounts in view of the patents previously cited. The process of this invention using dimethylsuccinate and a copper chromite catalyst can produce a significant amount of tetrahydrofuran depending upon the reaction temperature used.
U.S. Pat. No. 4,172,961 discloses a process for the production of 1,4-butanediol wherein a dialkyl alkoxy succinate-containing mixture is hydrogenated in the presence of a copper chromite catalyst to form a mixture comprising 1,4-butanediol and the corresponding alkanol. Among the by-products of the hydrogenation reaction are dibutylsuccinate, dibutylfumarate, 2-butoxy-1,4-butanediol and other higher molecular weight residues. Tetrahydrofuran is mentioned as a solvent but not as a product of the reaction.
U.S. Pat. No. 4,268,695 discloses a one-stage process for preparing butanediol by hydrogenating a solution of maleic anhydride in an alcohol in the presence of a copper chromite catalyst. Formation of small amounts of tetrahydrofuran is also disclosed but the yield of butanediol is said to be greater than 90 mole percent. Succinic esters are not disclosed as starting material but apparently appear among the by-products of the hydrogenation.
The separation of methanol from a mixture containing tetrahydrofuran, water and methanol using two columns at different pressures is disclosed in U.S. Pat. No. 4,332,645. The separation method of of the present invention is conducted with a greater amount of methanol. This is advantageous because the methanol/tetrahydrofuran azeotrope is easier to separate by superatmospheric distillation than the water/tetrahydrofuran azeotrope. Unlike the method of the patent, the separation method of the present invention results in complete purification of methanol because the methanol/tetrahydrofuran azeotrope taken overhead in the second tower is recycled to the first tower.
A one-step method of producing tetrahydrofuran and 1,4-butanediol from which significant amounts of either compound can be produced is more convenient and economical than a multi-step process which requires greater amounts of catalyst. Another advantage is the ability to efficiently vary the amounts of each material produced in response to market conditions or specific customer orders. The separation process is more efficient since the mixture of products is purified by one separation method.
The object of this invention is to provide a process of producing a mixture comprising tetrahydrofuran and 1,4-butanediol wherein the mole ratio of tetrahydrofuran to 1,4-butanediol is controlled by conducting the hydrogenation reaction within a particular temperature range. The presence of methanol during the reaction at the lower temperature range increases ester conversion and selectivity and reduces transesterification. A further object of this invention is a method of separating tetrahydrofuran and 1,4-butanediol from a mixture which also contains water and methanol.